Turkey has called for international backing for a cease-fire in the last rebel stronghold in Syria, telling the UN Security Council that an all-out assault on Idlib Province will result in a 'major humanitarian catastrophe.'

Air strikes and bombings will trigger a 'massive wave of refugees and tremendous security risks for Turkey, the rest of Europe, and beyond,' Turkish Ambassador Feridun Sinirlioglu told council members on September 11.

U.S. Ambassador Nikki Haley said Syrian forces, backed by Russia and Iran, had already launched 100 air strikes on Idlib this month and that their sole aim was 'a bloody military conquest' of the region.

Haley warned that 'the consequences will be dire' if the offensive goes ahead.

'The world will hold them responsible,' she said, adding that Russia 'has the power to stop the catastrophe looming' in Idlib.

Russia and Iran insisted that a military offensive in Idlib would be a counterterrorism operation and that measures will be taken to spare civilians.

'We cannot allow terrorists to hold hostage and use hundreds of thousands of people of Idlib as human shields,' Russian Ambassador Vasily Nebenzya said.

Iranian Ambassador Gholamali Khoshroo said the fight against terrorism in Idlib 'is an integral part of the mission of restoring peace and stability to Syria.'

Iran, Russia, and Syria often refer to any armed opponent of the Syrian government as a 'terrorist.' In the case of Idlib, an estimated 10,000 fighters with the Al-Qaeda-linked Hayat Tahrir al-Sham group are believed to be the dominant force among rebels in the province.

Moscow and Tehran have given Syrian President Bashar al-Assad's crucial support throughout the war, which began with a government crackdown on protesters in March 2011. Turkey is backing rebel groups and has troops in the country's north.

The 7 1/2-year war has killed more than 400,000 people and displaced millions.